This invention relates to a frequency selection circuit, and more particularly to frequency selection circuit which is equivalent to an LCR series resonant circuit, though not provided with inductance.
FIG. 1 shows a typical frequency selection circuit, or a so-called LCR series resonant circuit comprising a resistor 3 of a resistance value R, a capacitor 4 of a capacitance value C and an inductor or coil 5 of an inductance value L--all connected in series between two terminals 1 and 2. The circuit impedance Z between the terminals 1 and 2, resonant frequency f.sub.0 and selectivity or quality factor Q are represented as follows: ##EQU1##
In equation (1) .omega.=2.pi.f, where "f" denotes frequency, and in equation (3) .omega..sub.0 =2.pi.f.sub.0. As equations (1), (2) and (3) show, both L and C must be similarly varied in order to change the resonant frequency f.sub.0, without changing the quality factor Q. To change Q without changing the resonant frequency f.sub.0, it is necessary to vary R or to vary the ratio between L and C, while maintaining the product of L and C constant. In this case, if R is varied, the circuit impedance Z.sub.0 (=R) at resonance will change.
To change circuit impedance Z.sub.0 at resonance without varying resonant frequency f.sub.0 and quality factor Q, it is necessary to vary L, C and R. That is, at least L and C must be adjusted to vary resonant frequency f.sub.0, quality factor Q and circuit impedance Z.sub.0 at resonance independently of one another. But is extremely difficult to adjust L and C to a desired value. With such a frequency selection circuit as shown in FIG. 1 it is therefore very difficult to adjust resonant frequency f.sub.0, quality factor Q and circuit impedance Z.sub.0 at resonance, independently of one another.